Review Article ... · Advances in Tribology ... Review Article TribochemistryofIonicLiquidLubricantonMagneticMedia HirofumiKondo R&D ... persed in a polymer matrix with some additives

Hindawi Publishing CorporationAdvances in TribologyVolume 2012, Article ID 526726, 20 pagesdoi:10.1155/2012/526726

Review Article

Tribochemistry of Ionic Liquid Lubricant on Magnetic Media

Hirofumi Kondo

R&D Division, Sony Chemical & Information Device Corporation, 1078 Kamiishikawa, Kanuma 3228503, Japan

Correspondence should be addressed to Hirofumi Kondo, [email protected]

Received 13 February 2012; Revised 16 May 2012; Accepted 16 May 2012

Academic Editor: Arvind Agarwal

Copyright © 2012 Hirofumi Kondo. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The newly synthesized perfluoropolyether (PFPE) ionic liquid whose terminal group is an ammonium salt with a carboxylicacid has better frictional properties when compared to the conventional PFPEs. Stick-slip motion was not observed even for thesmooth surface for the modified PFPE tape. The friction is almost independent of the PFPE structure, but depends on the aminestructures. The ammonium salt being tightly anchored to the rubbing surface covers uniformly, which leads to better lubricity. Thehigher dispersive interaction of the hydrophobic group of the amine is endowed with a compensating friction reduction. Sterichindrance of the hydrophilic group causes a high friction. Based on these findings, a saturated long chain ammonium salt is thebest selection. Moreover, the modified PFPEs are dissolved in alcohol and hexane, which makes practical use convenient withoutany environmental problems. These ionic lubricants invented around 1987 have been used for magnetic tapes for about a quartercentury because of their good lubricity and are reviewed in this paper.

1. Introduction

Magnetic recording systems have been responsible for thewidespread and inexpensive recording of sound, video, andinformation processing. Despite the availability of othermeans of storing data, such as optical recordings andsemiconductor devices, magnetic recording media have theadvantages of low cost, stable storage, a relatively higher datatransfer rate, a relatively short seek time, and high volumetricstorage capacity [1].

2. Hard Disk Drive Systems (HDDs)

In current hard disk drive (HDD) systems, a rigid disk isrotated by a spindle motor at a speed of 10000 revolutions perminute (rpm). Information is written and read by a magnetichead with a tiny electric current attached to the end of theslider. The physical spacing between the magnetic sensorsand the disk is down to almost 10 nm in recent systems, andit will be necessary to be within 5–7 nm for areal densitiesin the Tbin−2 range [2]. The read/write magnetic heads aremounted in the slider and travel across the data zone duringthe reading and writing operations. However, when the drivestops, this head assembled device rests in the landing zone

which is typically textured in order to reduce wear duringcontact-start-stop (CSS) operations. Most drives require thatthe static and kinetic friction forces at the head-media-interface (HMI) remain low under extreme environmentalconditions and after the required number of CSS that isusually 10,000 or greater [3].

The rigid disk consists of an Al-Mg alloy or glass sub-strate, undercoat layer, a magnetic multilayer, a carbon over-coat, and a very thin lubricant layer as illustrated in Figure 1.Nowadays, the magnetic media are perpendicular media,which consist of a Co-Cr-based film [4]. A carbon overcoatis used to enhance the wear and corrosion resistance. Finallya molecular thin lubricant, which is the topic of this paper, iscoated to further reduce both the wear and stiction betweenthe HMI.

3. Tape Drive Systems

Magnetic tape media are divided into two categories [5];that is, particulate media in which magnetic particles are dis-persed in a polymer matrix with some additives and coatedonto the polyethylene terephthalate (PET) substrate, andthin film media in which monolithic magnetic thin films aredeposited onto the substrate in a vacuum, which is discussed

2 Advances in Tribology

CoCrPt

RuCoCr

Cr

RuAl

TiCr

Protective layer

Magnetic layer

Under layer

Substrate

15 nm

(a)

Carbon

Co and CoO

Magnetic layer

Protective layer

Substrate50 nm

(b)

Figure 1: Cross-sectional TEM image of magnetic layer for rigid disk (a) and magnetic tape (b).

in this chapter. For magnetic tape helical scan systems, thetape is driven by a pinch roller and a tension (0.2–0.5 N)is applied [6, 7]. Much higher data transfer rates can beachieved using this rotating head drive system [8]. The tapeused is a PET substrate with an evaporated film of Co whichis typically 100 nm thick. The magnetic layer fabricated withoxygen gas has obliquely aligned fine particles (Figure 1)[9, 10], which leads to higher electromagnetic characteristics[11, 12]. The carbon overcoat and the lubricant layer aredeposited onto the magnetic layer in a similar way to the rigiddisks [13].

The continuous demand for increasingly high recordingdensities has led to the development of monolithic magneticthin films. In the most recently established system, therecording density has increased about ten times in thisdecade using the highly sensitive magnetoresistive headsystems [14–16]. From the viewpoint of recording density,a smoother media surface exhibits a higher carrier to noiseratio, which makes the higher recording density possible.However, the smoother surface results in a higher real areaof contact and higher friction coefficient [17, 18].

4. Demand for Lubricant ofThin Film Magnetic Media

In conventional magnetic recording, thin film media typ-ically have their surfaces lubricated to reduce friction andwear resulting from contacts between the read/write mag-netic head and media surface. In practice, to avoid adhesionrelated problems, lubrication has to be achieved with amolecularly thin lubricant film [19]. However, the mainchallenge in selecting the best lubricant for a magnetic mediasurface is finding a material which provides wear protectionwhile the media surface is exposed to various environmentalsituations. It is important that the lubricants remain onthe media surface over the life of the file without beingsubject to desorption, spin-off, or chemical degradation.This problem has become more difficult with the advent of

a very smooth thin film surface, because thin film media donot have a mechanism for lubricant replenishment [20, 21].Furthermore, lubricant adhesion to the overcoat surfaceis often insufficient to prevent lubricant depletion thateventually results in accelerated wear.

The presence of an excess lubricant is often deemed nec-essary to replenish itself after sliding events. Increasing theamount of a lubricant enhances the durability, but exceedingthe surface roughness of the tape generally leads to adhesion-related problems, such as deleterious stiction. In order toreduce this trade-off, novel lubricants must be designed andsynthesized for the smoother surface magnetic thin filmmedia. The very large body of patents relating to the lubricityof perfluoropolyethers (PFPEs) on thin film magnetic mediashows the importance of this problem to manufacturers [22–27].

New types of PFPE lubricants whose chemical structureare summarized in Table 1 have been reported to enhancethe performance and reliability. Z-DOL has hydroxyl groupsat both chain ends, which has been widely used for the rigiddisk application. With the additional functional hydroxylgroups in the middle of the PFPE backbone chain, Z-tetraolmultidentate (ZTMD) can achieve a reduced clearance, whilestill achieving an overall drive reliability [28, 29].

The solid lubricants are used in high temperature andextreme high pressure environments, whereas liquid lubri-cants typically will not survive. Topical lubrication of solidlubricants such as graphite and molybdenum disulfide(MoS2) has not been successful because the solid lubricatinglayer is often found to interfere with the sensitive magnetictransducing process, and because most solid lubricants havea poor wear resistance, they tend to wear away in the tracksunder the head and generate debris [21]. Liquid lubricantshave the advantage that they will creep across the surface toreplenish a portion of the layer which has been removed byabrasion or head wear. However, because of their mobility,liquid lubricants may suffer the disadvantage of spinningoff from the disk surfaces during operation, especially

Advances in Tribology 3

Table 1: Functionalized PFPE lubricants [30].

X–CF2(OCF2CF2)n(OCF2)m OCF2–X (0.5 < n/m < 1)

Z X = –OCF3

Z-DOl X = –CH2OH

Z-DIAC X = –COOH

Z-tetraolX = –CH2 OCH2CHCH2 OH

OH

AM2001

–CH2OCH2X = O

O

CH2

at higher operating temperatures. These lubricants mayalso slowly evaporate with time at the high temperatures,thereby reducing their protection. The use of higher viscosity,low-volatility liquid lubricants may help to decrease theevaporation rate and prolong their life.

Over the past decade, ionic liquids have received a greatdeal of attention as a class of green solvents with a wide rangeof potential applications including organic and inorganicsynthesis [31], energy storage devices [32], separations [33,34], and catalysis [35–37]. The term ionic liquid is broadlyused to describe a large class of low melting fused saltsthat are liquids below 100◦C. The most notable character-istics of many ionic liquids are their low vapor pressure,nonflammability, thermal stability, wide liquid range, andsolvating properties for diverse substances. Limited resultsfrom very recent studies have shown the potential for usingionic liquids as a new class of lubricants. Friction and wearreductions have been reported on metallic and ceramicsurfaces lubricated by selected ionic liquids compared to theconventional hydrocarbon lubricants [38, 39]. Ammonium-based ionic liquids provide friction reduction from elasto-hydrodynamic to boundary lubrication regimes comparedto the fully-formulated base oil [40]. Ionic liquids have alsobeen studied to determine their effectiveness as additivesfor base oil and water, and the chemical and tribochemicalreactions have been evaluated to understand the lubricationmechanisms [41–44].

Ionic liquids, which possess an octadecyl ammonium saltwith pentadecafluoro octanate, significantly reduce the fric-tion compared to the corresponding amide and Z-DOL [45–50]. The modified PFPEs having the same hydrophilic grouphave also been synthesized and also show better frictionalproperties, which have been used as a lubricant for magneticthin film media for a long time [51–53]. This type of ionicliquids are named protic ionic liquids which are a subsetof ionic liquids formed by the stoichiometric (equimolar)combination of a Bronsted acid with a Bronsted base [54–57]. Relevant investigations into the molecular interactionsof carboxylic acids and amines were conducted by Kohler etal., and the complexes of acid and amine with the molecularratio of 1 : 1 can be found [58, 59]. In this paper, a series ofionic lubricants having the same hydrophilic group statedabove are deposited on the magnetic thin film media andthe effect of their molecular structures on the frictionalproperties is systematically investigated.

The lubricant is required to be very thin on the order ofa monomolecular layer. Therefore, the frictional propertiesdepend not only on the molecular structure, but also onthe microscopic structure of the lubricant film [60, 61].Microscopic coverage of this alkylammonium-based proticion liquid film on the medium surface is also examinedusing FTIR and X-ray photoelectron spectroscopy (XPS) andrelated to the spectra to the frictional properties.

5. Materials

Three types of lubricants which possess both the perfluo-roalkyl group and long chain hydrocarbon, that is an ester,amide, and carboxylic acid ammonium salt, were synthesizedby the following Scheme 1 in Figure 2. The ester and theamide were prepared by the addition of carboxylic acidchloride to the hexane solution of the corresponding alcoholand amine in the presence of a base agent. The perfluoro-carboxylic acid ammonium salts are prepared by warmingthe mixture of the perfluorocarboxylic acid and the amine to80◦C until the complete dissolution was obtained (Scheme 2)[45, 47]. The ammonium salts of long chain fatty acid weresynthesized in the same manner (Scheme 2). They are thenrecrystallized from n-hexane.

The ammonium salts with PFPE carboxylate lubricantswere synthesized according to Scheme 2 and Scheme 3in Figure 2 by merely warming the mixture of the abovecarboxylic acid and a 5% excess of the long chain alkyl amineto 80◦C with stirring until complete dissolution is obtained.Three different types of PFPEs, which possess a carboxylicacid group as the end group, are used as the raw materials.K-lubricant is a homopolymer of perfluoro-isopropyleneoxide, and D-lubricant is a homopolymer of perfluoro-n-propylene oxide. K-lubricant and D-lubricant have one endgroup. Z-lubricant is a random copolymer of the perfluorooxymethylene and oxyethylene oxide monomers, which havetwo identical end groups. The average molecular weight ofthe PFPEs is about 2000. Since most commercial PFPEshave a fairly broad and asymmetrical molecular weight dis-tribution, a small excess of the alkyl amine is used, whichis removed by washing with n-hexane after the reaction[52]. The chemical structure is determined by its infraredspectra: 3200–2800 cm−1 (N+H3 stretching), 2918 cm−1 and2958 cm−1 (CH2 stretching), 1674 cm−1 (CO stretching),1280–1110 cm−1 (CF stretching). The CO stretching movedfrom 1800 cm−1 to 1674 cm−1, and the N+H3 stretching at3200–2800 cm−1 appears, thus identifying the ammoniumsalt with a carboxylate structure. The synthesized lubricantsare summarized in Table 3. Each lubricant was deposited ona magnetic layer by a dip-coating method.

6. Friction Properties

6.1. Friction Measurement Apparatus. The apparatus shownin Figure 3 was used to measure the CSS friction charac-teristics of the rigid disks. Friction at the head slider wasmeasured by a strain gauge for each CSS operation duringthe starting of the spindle motor with a 10 g load at 25◦C,50% relative humidity.


Scheme 2

Scheme 3

Scheme 1

CF3

, C6H5

Rf = F–(CF2CF2CF2O)n–CF2CF2– for D–lube

Rf = F–(CF2CFO)n–CF2– for K–lube

Rf = F–(CF2)m– m =7, 9

Rf = F–(CF2)m– m =7, 9

R1 = CnH2n+1,2n−1

Rf –COCl + R1–XH

Rf –COOH + R1–N(R2)2 Rf –COO−N+H(R2)2R1

–

R1,R2 = CH3, CnH2n+1,2n−1,2n−5

CH3, CnH2n+1,2n−1,2n−5

HOCO–Rf –COOH + R1–N(R2)2

–CF2O–(CF2O)m–(CF2CF2O)n–CF2O–CF2– for Z–lube

Rf –COXR1

X = O, NH

Rf = CnH2n+1,2n−1,2n−5

R1, R2 =

Rf =(R2)2R1N+HO−CORf –COO−NH+(R2)2R1

Figure 2: Synthetic scheme for the new ionic liquid lubricant and the reference compound.

Head sliderStrain gauge

Motor

Rigiddisk

Disk

Slider

Straingauge

Suspension AE transducer

Figure 3: Friction measuring apparatus for rigid disk.

A schematic diagram of the friction measurement appa-ratus for the magnetic tapes is shown in Figure 4. Thecoefficients of kinetic friction are measured for 8-mm widetapes sliding around a quadrant of a 4-mm diameter polishedstainless steel (SUS 304) cylinder. The friction coefficient wascalculated from the change in the sliding of the tension (T1)exerted by a 20-g weight (T2) hanging from the tape slidingon the cylinder. A 50-mm section of the tape is made to slideagainst the cylinder at a speed of 5 mms−1 in a reciprocatingmotion at 25◦C, 60% relative humidity.

6.2. Frictional Performance of the Newly Synthesized Ionic

Lubricant with Ammonium Salt for Magnetic Media

6.2.1. Rigid Disks. The new lubricants exhibited a high per-formance compared to the conventional PFPE (Z-DOL) asshown in Figure 5. Frictional coefficient of disk coated with

Stainless steel cylinder

Strain gauge

Screw drive

Tape

Motor

Load

T1

T2

Figure 4: Schematic diagram of the friction measurement appara-tus for ME tapes.

0 0.5 1 1.5 2

0.2

0.4

0.6

0.8

0

Number of CSS operations

Fric

tion

coe

ffici

ent Perfluoropolyether (Z-DOL)

Ammonium salt(Lubricant 10)

×104

Figure 5: Frictional coefficient of disk coated with lubricant (10)versus number of CSS operations. The functional PFPE (Z-DOL)shown for comparison.


Table 2: Molecular structures and melting points of ionic liquid lubricants RF–CO–Y–R.

No. RF Y R mp/◦C Remark

1 C7F15 O C18H37 29

2 C7F15 O C18H31 <20

3 C7F15 NH C18H37 91

4 C7F15 NH C18H31 48

5 C7F15 O−H3N+ C18H37 55

6 C7F15 O−H3N+ C18H31 <20

7 C9F19 O−H3N+ C12H25 61

8 C9F19 O−H3N+ C14H29 65

9 C9F19 O−H3N+ C18H37 71

10 C9F19 O−H3N+ C24H49 85

11 C9F19 O−H3N+ C18H35 26

12 C9F19 O−H3N+ C18H31 <20

13 –CF2O–(CF2O)m–(CF2CF2O)n–CF2O– O−H3N+ C4H9 <30 Z-lubricant






19 –CF2O–(CF2O)m–(CF2CF2O)n–CF2O– O−H3N+ C16H33 < 30 Z-lubricant

20 –CF2O–(CF2O)m–(CF2CF2O)n–CF2O– O−H3N+ C18H37 38–40 Z-lubricant

21 –CF2O–(CF2O)m–(CF2CF2O)n–CF2O– O−H3N+ C20H41 58–61 Z-lubricant



24 –CF2O–(CF2O)m–(CF2CF2O)n–CF2O– O−H2N+(CH3) C18H37 79–82 Z-lubricant

25 –CF2O–(CF2O)m–(CF2CF2O)n–CF2O– O−HN+(CH3)2 C18H37 55–57 Z-lubricant

26 –CF2O–(CF2O)m–(CF2CF2O)n–CF2O– O−H2N+(C18H37) C18H37 51–55 Z-lubricant

27 –CF2O–(CF2O)m–(CF2CF2O)n–CF2O– O−H2N+(C6H5) C18H37 31–33 Z-lubricant

28 F–(CF2CF2CF2O)n–CF2CF2– O−H3N+ C4H9 <30 D-lubricant








36 F–(CF2CF(CF3)O)n–CF2– O−H3N+ C18H37 <30 K-lubricant

37 C17H35 O−H3N+ C18H37 92

38 8-C17H33 O−H3N+ C18H37 47

39 16-C17H33 O−H3N+ C18H37 84

40 8, 11, 14-C17H29 O−H3N+ C18H37 40

Table 3: Structure of the lubricant.

Molecular structure mp/◦C

Lubricant 7 C9F19 O−H3N+ C12H25 61




lubricant (10) versus number of CSS operations is shownand the conventional functional PFPE (Z-DOL) shown forcomparison. The relationship between the CSS durability

and the molecular structure of the lubricant in terms ofthe polar group, chain length, and chain symmetry wasinvestigated.

6.2.2. Magnetic Tapes. The frictional characteristic of thecarboxylic acid ammonium salt coated on the magnetic tapeby dip-coating is shown in Figure 6. The friction coefficient isshown as a function of the number of cycles of reciprocatingmotion over the cylinder. The frictional characteristic of aPFPE (Z-DOL) is shown for comparison. For the ammo-nium salt, the friction coefficient is low and remains at 0.18


0 10 20 30 40 50 60 70 80 90 100

0.5

0.4

0.3

0.2

0.1

0

Z-DOL

Ammonium salt(Lubricant 9)

Number of reciprocating motions

Fric

tion

coe

ffici

ent

Figure 6: Friction coefficient of the carboxylic acid ammonium salt(Lubricant 9) and Z-DOL during friction test at 25◦C and relativehumidity of 60%.

1 10

Fric

tion

coe

ffici

ent

0.2

0.4

0.6

0.8

1

0


EsterAmide

Ammonium salt

102 103 104

Figure 7: Friction coefficient of disk coated with ester lubricant(Lubricant 1), amide (Lubricant 3), and carboxylic acid ammoniumsalt (Lubricant 5) versus number of CSS operations.

even after 100 cycles of reciprocating motions, but it is morethan 0.30 for the PFPE.

7. Effect of Molecular Structure on Friction

7.1. Hydrophilic Group

7.1.1. Effect of Hydrophilic Group on CSS Friction. The CSSfriction properties of three types of lubricants, that is, ester(Lubricant 1), amide (Lubricant 3), and carboxylic acidammonium salt (Lubricant 5) are shown in Figure 7. Thesefriction measurements of the synthesized lubricants revealedthat the ester and the amide are far less durable than thecomparable salt type. For the ester and amide lubricants,the friction coefficients (μ) are around 0.25 for the first tenCSS operations, but rise with the increasing number of CSSoperations (n). Especially, for the ester lubricant, μ steeplyincreases after 20 operations, and the carbon protective layer

Fric

tion

coe

ffici

ent

0 10 20 30 40 50

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Amide

Ammonium salt

Fluorine atomic (%)

Figure 8: Relation between the relative intensity of fluorine atomon the tape surface and friction coefficient after 100 cycles ofreciprocating motions.

gets damaged when the μ became over 0.90. The μ of theamide lubricant increased gradually and a wear scar occurredat 3279 operations. For the carboxylic acid ammonium saltlubricant, the μ value remained nearly constant at around0.25 throughout the 104 CSS operations and the mediumwas scarcely damaged. The low initial value of μ, 0.2–0.3,indicates that there is sufficient lubricant film to protect therubbing surface.

7.1.2. Frictional Tests for the Lubricant with a Different PolarGroup for Magnetic Tapes. The relation between the frictionafter 100 reciprocating cycles and the amount of thelubricant on the magnetic surface is shown in Figure 8. Theamount of lubricant on the surface can be varied with thelubricant concentration of dip-coating solution. Clearly, theammonium salt gives a better frictional characteristic thanthe corresponding amide. For the salt, the friction coefficientdecreases with the increasing lubricant on the surface andreached 0.18. In contrast, the friction coefficient of the amideis almost independent of the amount of lubricant and is veryhigh (approximately 0.45). These results revealed that thelubrication mechanism of the salt and the amide are differentand depend on the polar group.

The frictional properties for the two lubricants dependon both the lubricant polar group and the surface concen-tration, but not the nonpolar hydrophobic groups, sincethe two had identical hydrophobic groups. Therefore, acomparison of the polar group effects on friction coefficientneeds to be made with nearly the same amount of eachlubricant on the surface. The fluorine content was measuredby XPS and the film thickness was calculated [62–64]. Theselected amount gave a relative intensity of the fluorinesignal of about 40 atomic % in the XPS measurements,


20 40 60 80

0.1

0.2

0.3

0.4

0.5

0.6

10000

0.7

Number of reciprocating motions

Fric

tion

coe

ffici

ent

Amide

Ammonium salt

Figure 9: Comparison of frictional properties of ammoniumcarboxylate ionic liquid and the corresponding amide during afriction test with 40 atomic % fluorine.

Fric

tion

coe

ffici

ent

0.2

0.4

0.6

0.8

1

012 14 16 18 20 22 24

Number of carbon atoms of amine molecule

Lubricant 7

Lubricant 8

Lubricant 9

Lubricant 1060

70

80

90

100

50

Mel

tin

g po

int

(◦C

)

Figure 10: Friction coefficients of the salt type lubricants after20,000 CSS operations.

since this is approximately the intensity of a monolayer forthe salt prepared by the Langmuir-Blodget method [59].This corresponds to the concentration of 0.26 mmol L−1 and1.27 mmol L−1 of the chlorofluorocarbon solution for the saltand for the amide, respectively.

Figure 9 shows the friction coefficient variation versus thereciprocating cycles for the salt and for the amide at the aboveconcentration. For the salt, the friction coefficient valueremained low and steady at approximately 0.23 throughoutthe 100 reciprocating cycles. The friction coefficient with theamide increased with the number of cycles. The amide showsthis increase in friction at all the concentration.

7.2. Hydrophobic Group

7.2.1. Effect of Alkyl Chain Length on CSS Friction. Thehydrocarbon chain lengths of the salt type lubricant werechanged and the CSS durability measured (Figure 10). Thestructures of the lubricant are shown in Table 3. As the num-ber of carbon atoms in the amine molecule (n) increased, μdecreased until a nearly constant value of 0.24 was attained.

1 10

Fric

tion

coe

ffici

ent

0.2

0.4

0.6

0.8

1

0


Linolenyl ammonium salt(Lubricant 12)

Oleylammonium

salt(Lubricant 11)

Stearyl ammonium salt(Lubricant 9)

102 103 104

Figure 11: Friction properties of the salt type lubricant of longchain C-18 amine.

The shorter homologues showed an increase in μ as a resultof the breakdown of their film [65].

For the given polar group, one of the key propertiesrequired for friction reduction is a high intermolecular cohe-sion energy (due to dispersive or van der Waal’s interactions)between the hydrocarbon chains. Melting of the lubricantmaterials by heating involves disruption of the dispersiveinteractions between the hydrocarbon chains [66], therefore,the melting point of the lubricant should be related to thedispersive interactions of the hydrocarbon chain [64]. Themelting points of the lubricants with different chain lengthslisted in Table 2 are also plotted.

The melting point becomes higher with the increasinghydrocarbon chain length and μ decreases. Not only the polargroup, but also the chain length due to dispersive interactionsdetermined the durability.

7.2.2. Chain Symmetry (Double Bond Effect). The unsat-urated oleyl (Lubricant 11) and linolenyl (Lubricant 12)ammonium salts used for a comparison with the saturatedstearyl ammonium salt were synthesized, which have thesame polar group and the same chain length. The oleyl aminehas one double bond and the linolenyl amine has three. TheCSS properties are shown in Figure 11.

Highly symmetrical (straight) molecules can be morereadily arranged than the less symmetrically constituted(bent) molecules. Since high packaging in the lubricantlayer is more favorable for highly symmetrical molecules,the symmetrical lubricants generally have higher cohesiveinteractions than their less symmetrical counterparts. As thesalt type lubricants are strongly adsorbed on the carbonlayer, we can show a model for the salt type lubricants inFigure 12. Saturated chains (e.g., stearyl ammonium salt) arelinearly symmetrical and can efficiently pack. However, theunsaturated chains, particularly the cis-conformation chains(as in the oleyl and linolenyl ammonium salts), are bent, andless symmetrical, therefore, they do not pack well. Thus, thedouble bond ammonium salt has a low melting point andshows an increase in μ already mentioned.


Hydrophilic group Magnetic layer

Hydrophobicgroup

C

Cohesive interaction

18H37

C9F19

C18H35

C18H31

Figure 12: A model for the salt type lubricants adsorbed on the carbon layer.

Number of reciprocating motion

0.09

0.18 0.27

0.45

0.72

0.4

0.5

0.6

0 10 20 30 40 50 60 70 80 90 100

0

0.1

0.2

0.3

Fric

tion

coe

ffici

ent

mmol L−1 mmol L−1

mmol L−1 mmol L−1

mmol L−1

Figure 13: Concentration effect on friction of the lubricant solu-tion.

7.3. Long Chain Hydrocarbon Carboxylic Acid

Ammonium Salt

7.3.1. Effect of Concentration of Dip-Coating Solution on Fric-tion. The frictional properties of the stearic acid stearylammonium salt (37) as a function of the dip-coating concen-tration are shown in Figure 13. The initial friction coefficientat the concentration of 0.09 mmol L−1 is 0.25, and it increaseswith the number of reciprocating cycles. The lowest initialfriction coefficient is observed at the concentration of0.18 mmol L−1 increased with the higher lubricant concen-tration, and it became very high at 0.72 mmol L−1.

The concentration of 0.27 mmol L−1 results in a thicknessof approximately one monolayer for the lubricant which hadthe carboxylic acid ammonium salt as a polar group [46].The increase in friction with the increasing reciprocatingcycles seems to indicate that the surface of the magneticlayer is not sufficiently covered by the lubricant below thatconcentration.

On the contrary, since excess lubricant at the surfacewould result in a higher adhesional friction, the initial valuebecame higher with the higher lubricant concentrations.According to the meniscus theory, friction should increasein the case of smooth surface with an increase in the lubri-cant thickness. The friction of lubricated media generallyincreases if the lubricant thickness is increased with respectto media roughness and creates menisci around individualasperity contacts [67]. A decrease in friction with the increas-ing reciprocating cycles reveals that an excess of lubricanttransfers to the stainless steel cylinder counterface. Forconcentrations above 0.27 mmol L−1 the friction coefficientapproaches the value for the 0.27 mmol L−1 concentration.

7.3.2. Number of Double Bond and Its Position Effect on Fric-tion. In order to investigate the effect of the number ofdouble bonds and their position in the molecular structure,the following three such lubricants were tested: 8-oleic acid(Lubricant 38) ammonium salt, 16-oleic acid (Lubricant39) salt, and linolenic acid (Lubricant 40) salt which havethree double bonds at positions 8, 11, and 14, respectively.Figure 14 reveals the friction coefficient variations versusreciprocating cycles for four lubricants at the concentrationof 0.27 mmol L−1. The introduction of a double bondcauses an increase in friction with the number of cycles,and particularly for the linolenic acid ammonium salt, thefriction coefficient steeply increases. For the oleic acid salt,the friction coefficient slightly increases with the numberof double bonds in the hydrophobic group and the frictioncoefficient is higher for a double bond at the terminalposition-16 compared to the center position-8.

The unsaturated chain, particularly the cis-conformationchains in the oleic acid and linolenic acid, are bent and,hence, they do not pack well. Therefore, with more doublebonds, the melting point becomes lower, as demonstratedin Table 2, and also shows an increase in the frictioncoefficient. However, this does not explain two observations:(1) the friction coefficient increases with the number ofreciprocating cycles, and (2) the lubricant with a double

Advances in Tribology 9Fr

icti

on c

oeffi

cien

t

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7


0 20 40 60 80 100

Linolenic acidsalt

16-oleicacid salt

8-oleicacid salt

Stearic acidsalt

Figure 14: Effect of the number and position of the double bondson the friction coefficient at a concentration of 0.27 mmol L−1.

Load

PinTape sample

MovedGlass plate

Strain gauges

Figure 15: Schematic diagram of pin-on-flat friction tester.

bond at position 8 (the center) has a lower friction value thanthat at the terminal position 16.

In this case, the lubricant had a carboxylic acid ammo-nium salt as a polar group, which was strongly adsorbed onthe tape surface compared to the double bond. This oleopho-bic group in the lubricant proved difficult to interact with themagnetic surface, but easily interacted with the stainless steelcylinder counterpart. This may help to explain why 16-oleicacid with a terminal double bond has a higher friction coef-ficient than the lubricant with a double bond at position 8.

7.3.3. Effect of Double Bond on Wear. The tapes treated withthe linolenic and stearic acid ammonium salts were alsotested by a pin-on-flat apparatus in Figure 15. The measuredfriction coefficient values versus the number of reciprocatingcycles are shown in Figure 16. The test was terminated whenthe magnetic layer became damaged, except at the load of2 g in which the tapes are scarcely damaged even after 50reciprocating cycles. Again, all the lubricants with doublebonds resulted in an increase in the friction, although thetest geometry used here is much different from the tape testerused in the previously discussed results.

The lubricants having double bonds showed no obviouswear in the friction region where the tape coated with thenondouble bond lubricant was damaged. For example, the


0 10 20 30 40 500

1

2

3

4

5

6

7

8

9

10

11

9,12,15-linolenylStearyl

Fric

tion

forc

e (g

)

20 g

10 g

5 g 2 g

Figure 16: Change in friction force of stearic acid and linolenic acidsalts in pin-on-flat tests.

nondouble bond lubricant required six reciprocating cyclesat the load of 10 g to display damage and the frictionalforce of 3.1 g. However, the double bond lubricants had notbeen scarred until the frictional force reached 4.2 g at theeighth reciprocating cycle. This indicates that the doublebond lubricant behaves like a wear protective film and hasa higher load carrying capacity.

In order to characterize the surface of the magneticlayer from the tribological experiments, an FTIR-reflection-absorption spectroscopy (RAS) analysis was completed. TheFTIR method allows chemical information to be obtainedon the molecular level thickness, including the molecularstructure.

The FTIR spectra in the 3000 cm−1 to 2800 cm−1 regionbefore and after the friction tests are shown in Figure 17.The complete spectra of materials are not shown in this case.The peaks were shifted from 2916 cm−1 and 2850 cm−1 forthe bulk (uncoated) lubricants to 2926 cm−1 and 2854 cm−1,respectively, as shown in the spectra of Figures 17(a) and17(c). For each lubricant film, these peaks are assigned tothe CH2 asymmetric and symmetric stretching vibrations.Nevertheless, the melting points of the bulk materials arehigher than the ambient temperature, and this higherfrequency shift reveals that the alkyl chain in the liquid phaseor in a solution where the cohesive interaction between themis weak [68].

For the stearic acid salt film, the intensities of the spectraboth before and after the friction test are similar, indicatingthat the rubbing motion did not cause a change in themicroscopic structure of the film, such as the thickness andfilm formation. On the other hand, the intensities of the CH2

stretching vibration became much weaker after the frictiontest for the linolenic acid film, which reveals that the thick-ness of the film was reduced due to rubbing. The decreasein film thickness resulted in a higher friction coefficient. Forthe pin-on-disk tests, the slight protective effect from double


Abs

orba

nce

0.1%

27202800288029603040

(a)

(b)

29262854

0.1%

(a)2926

2854

2728002880296040

(b)

Stearic acid ammonium salt film

Wavenumber (cm−1)

(a)

27202800288029603040

0.1%

Abs

orba

nce

(c)

(d)

2854

2926

Linolenic acid ammonium salt film

Wavenumber (cm−1)

(b)

Figure 17: FTIR spectra of before and after the test. (a) RAS ofstearic acid ammonium salt film, (b) RAS of film after the test. (c)RAS of linolenic acid ammonium salt film, (d) RAS of film after thetest.

bond lubricants seems to reveal the existence of a polymerfilm, but this has not been proven.

7.4. Frictional Properties of Modified PFPEs on Magnetic Tapes

7.4.1. Performance of New Lubricant. The good viscositycharacteristics, low melting point, low surface energy, lowvolatility, and good thermal stability of PFPE are among theimportant criteria for selecting a lubricant. By changing theperfluorocarboxylic acid with PFPE carboxylic acid, that isZ-lubricant in Scheme 3 and D- and K-lubricants in Scheme2 of Figure 2, the modified PFPEs are expected to have lower

Number of cycles of reciprocating motion

0

0.1

0.2

0.3

0.4

0.5

Modified D-lube (Lubricant 35)Modified K-lube (Lubricant 36)

Modified Z-lube (Lubricant 20)

Z-DOL

20 40 60 80 100

Fric

tion

coe

ffici

ent

Figure 18: Frictional properties of the three types of the modifiedPFPEs using stearyl amine. The conventional Z-DOL is shown forcomparison.

Magnetic layer

Protective layer

PET substrate SiO2 particles (8–18 nm)

Figure 19: Schematic diagram of the cross section of a magnetictape.

melting points and improved thermal properties comparedto the corresponding perfluorocarboxylic acid homologue.

The frictional characteristics of the three types of mod-ified PFPEs of the ammonium salt with carboxylate areshown in Figure 18. For the ammonium salt, it is low andapproximately 0.17 even after 100 cycles of reciprocatingmotion, and is not dependent on the chain structure of thePFPE. However, it is over 0.30 for the conventional PFPE,and other types of end groups, such as the other hydroxyland piperonyl, have a similar frictional coefficient of approx-imately equal to or greater than 0.30 (data not shown).

7.4.2. Friction on Smoother Surface. In order to design thesurface morphology, the smaller spherical SiO2 particles arecoated onto the substrate with a polymer binder solutionbefore the magnetic layer is deposited (Figure 19). Thesurface asperity can be controlled by changing the size of theSiO2 particles to compromise the trade-off of electromag-netic characteristics and durability [69–71].

Most contacts in magnetic media are elastic; therefore,the kinetic frictions are expected to be higher for tapes witha lower surface roughness. The surface roughness (Ra) ofthe tapes with the surface SiO2 particle diameters of 8, 12,and 18 nm were measured using an optical profiler, andare 3.3 nm, 1.7 nm, and 1.4 nm, respectively. The frictioncoefficients for the carbon coated tapes of different surfaceroughness are shown in Figures 20(a), 20(b), and 20(c).


0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

AM

DOL

DIAC

Amine


Fric

tion

coe

ffici

ent

(a)

AM

DOL

DIACAmine

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100


Fric

tion

coe

ffici

ent

(b)

AM

DOLDIAC

Amine

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100


Fric

tion

coe

ffici

ent

(c)

Figure 20: Frictional properties of each PFPE based on the different surface roughness using (a) 8 nm, (b) 12 nm, and (c) 18 nm particles.

The thickness of the lubricant was almost the same valueof 1.2 nm for each PFPE tape. The friction coefficients werestable for the tapes with the 12 nm and 18 nm particles, butwere unstable at the beginning (several ten reciprocatingmotions) for the smoother 8 nm tapes. The differences infriction for each PFPE tape were then compared: AM, Z-DOL, Z-DIAC, and ammonium salt (the friction coefficientdecreases in that order). The difference is greater for thesmoother surface.

Figure 21 shows the dynamic friction coefficient duringthe reciprocating operation for each PFPE in the case of the12 nm tapes. The amplitude of the saw tooth pattern in thefriction curve is significantly high for the AM and the Z-DOLtapes; these fluctuations in sliding resulted from the stick-slipprocess and are associated with squeal and chatter. However,the dynamic friction coefficient was relatively constant andthe stick-slip phenomenon is only slightly observed for theZ-DIAC and the ammonium salt tapes.

7.4.3. Effect of Molecular Length of Amine. In order to exam-ine the effect of the amine structure, the friction coefficientsare also measured for the tapes versus the hydrocarbon chain

length of the amine. The relation between the number ofcarbon atoms and the frictional coefficients after 100 cycles ofreciprocating motion for the modified Z- and D-lubricantsis shown in Figure 22. As the number of carbon atoms in theamine molecule increases, the frictional coefficient decreasesto a nearly constant value of 0.17 in both cases when thenumber of carbons exceeds 14.

7.4.4. Effect of Molecular Structure of Amine. Secondly, themolecular structure of the amine is changed using stearylamine derivatives in order to fix the chain length of thelongest substituent. The frictional results of the PFPEammonium salts after 100 reciprocating motions and theirmelting points are summarized in Table 4. The introductionof a double bond into the hydrocarbon long chain, an oleylwith one double bond and a linolenyl with three, also makesthe friction coefficient increase.

By replacing the hydrogen atoms of the amine groupwith alkyl and phenyl groups, a different series of salts wouldbe obtained. This trend strongly suggests that replacing thehydrogen atom of the amine with an electron donatingalkyl group increases the electrostatic interactions (including


0.3

0.2

0.1

00 1 2

Distance (cm)

Fric

tion

coe

ffici

ent

(a) Ammonium salt

Distance (cm)

0.4

0.3

0.2

0.1

00 1 2

Fric

tion

coe

ffici

ent

(b) Z-DIAC

Distance (cm)

0.5

0.4

0.3

0.2

0.1

0.6

00 1 2

Fric

tion

coe

ffici

ent

(c) Z-DOL

Distance (cm)

0.5

0.4

0.3

0.2

0.1

0.6

0

0.7

0.8

0 1 2

Fric

tion

coe

ffici

ent

(d) AM

Figure 21: Change in dynamic friction coefficient during the reciprocating operation for the different PFPEs.

Number of the carbon atoms of amine molecule

Fric

tion

coe

ffici

ent

0.1

0.2

0.3

0.4

12 14 16 18 20 22864 10

Modified Z-lubeModified D-lube

Figure 22: The relation between the number of carbon atoms andfrictional coefficient after 100 cycles of reciprocating motion.

possible hydrogen bonding) between the cation and anionin the salts, which, in turn, raises the melting point. Thealkyl substituted amine, for example, by methyl and stearylgroups, have higher melting points than the nonsubstitutedstearyl amine, nevertheless, the friction coefficient was some-what inferior to the stearyl amine. Exchanging the amine ofthe salts for the larger phenyl substituent generally produceda further decrease in the melting point. As a general trend,for each anion, the salts with cations of lower symmetryshow a lower melting point than those with cations of highersymmetry. Also, adding the bulky phenyl group significantlyincreased the friction. These magnitudes of increase in thefriction suggest a blocking effect by the large substituentattached to the amine nitrogen, which hinders adsorption ofthe polar group on the media surface. Steric hindrance by thepolar group caused the high friction.

The cohesive energy density is normally lower for thefluorocarbon and ether group compared to the hydrocarbon.Cong et al. indicated that the film strength controlled by


Table 4: Fiction coefficients and melting points of the lubricants with the molecular structure of the amine having 18 carbon chains.

Lubricant number Structure of lubricant Friction coefficient Melting point/◦C

20 (stearyl) C18H37 NH2 0.17 38–40

22 (oleyl) C18H35 NH2 0.20 <30

23 (linolenyl) C18H31 NH2 0.25 <30

24 (methyl stearyl) C18H37 NHCH3 0.19 79–82

25 (dimethyl stearyl) C18H37 N(CH3)2 0.20 55–57

26 (distearyl) (C18H37)2NH 0.21 51–55

27 (phenyl stearyl) C18H37 NHC6H5 0.30 31–33

intermolecular attractive forces is an important factor thataffects the frictional properties of the monolayers, which isassociated with a higher load-capacity [72]. The attractiveforces between the fluorocarbon chains are lower than thosebetween the hydrocarbon chains [73] and the presenceof oxygen atoms in the PFPE backbone decreases theintermolecular attractive force [74–76]. For the ammoniumsalts of the PFPEs, the reason for the lower friction mightbe the incorporation of a hydrocarbon into the molecules.The longer the hydrocarbon chain length, the lower thefriction coefficients become as shown in Figures 10 and 22.Therefore, it is expedient to increase the dispersive interac-tion by introduction of a hydrocarbon chain into a PFPEmolecule without steric hindrance between the polar groupof the lubricant and media surface. To be precise, a saturatedstraight long chain ammonium salt is the best selection.

The use of a conventional PFPE is limited by the solvent.However, since the modified PFPEs contain a hydrocarbonmoiety and ammonium salt moiety, it is soluble in alcohols,and other conventional fluorinated solvents, which makes itspractical use convenient. Figure 23 shows the consequencesfor friction of the stearyl ammonium salt of Z-lubricant bychanging a thinner of dip-coating. In this case, ethanol, n-hexane-20% wt. ethanol, and a fluorinated solvent were used.It is evident that the frictional properties were independentof the dip-coating solvent.

8. Langmuir-Blodgett (LB) Films ofthe Salt Type Lubricant

In order to elucidate the molecular level structure of thespontaneously adsorbed layers, a comparative structuralstudy with films prepared by the Langmuir-Blodgett (LB)method is useful [77]. Organized ultra thin films of con-trolled thickness are deposited on solid substrates by the LBtechnique. The effectiveness of LB films in protecting themagnetic thin film media was reported [78]. A stable andclosed packed monolayer film can be obtained at the air-water interface using Joyce-Loebl trough. These films weretransferred onto the magnetic media by vertical dippingmethod. Langmuir films (L films, i.e., monolayer on thewater surface) and LB films of the ammonium salt lubricantswere prepared and studied.

8.1. Basic Properties of the L Films and LB Films. The ammo-nium salt with perfluorocarboxylate (Lubricant 5), and the

0 20 40 60 80 100

Number of cycles of reciprocating motions

Fric

tion

coe

ffici

ent

0

0.1

0.2

0.3

0.4

0.5

Hexane-20% wt. ethanolEthanol

HF2-(OC2F4)p-(OCF2)q-CF2H

Figure 23: Influence of changing a thinner of lubricant on thefriction coefficient.

corresponding ester (Lubricant 1) and the amide (Lubricant3) are compared. The salt forms a much more stable mono-layer on the water surface than the other two ester and amidelubricants. Figure 24 shows the area decay curve of the L filmsof the salt and amide on the water surface when they are heldat a surface pressure of 25 mNm−1. The curve for the ester isunstable such that it collapsed as it was compressed. The areadecay over 1 hour was less than 5% for the salt, whereas forthe amide it was 45%, indicating a good stability for the saltmonolayer. This result suggests a good balance between thepolar and hydrophobic nature of the salt molecule, which is anecessary condition for producing a stable monolayer on thewater surface.

Furthermore, the isotherm of the salt shown in Figure 25suggests that the molecules of the salt were closely packedin the L film. From Figure 25, the area per molecule at25 mNm−1 is about 0.6 nm2. The areas occupied by an alkylchain and a perfluoroalkyl chain are about 0.2 nm2 and0.4 nm2, respectively, when their chain axes are perpendic-ular to the water surface. Therefore, it is suggested that thealkyl chains and perfluoroalkyl chains of the salt moleculesare highly ordered and closely packed with their chain axesperpendicular to the water surface in the L film.

The closely packed monolayer is transferred onto thesurface of the magnetic layer at 25 mNm−1 with a dipping


0 10 20 30 40 50 6050

60

70

80

90

100

Frac

tion

al a

rea

rem

ain

ing

(%)

Time (min)

(a)

(b)

Figure 24: Area decay curve of the L films of the salt (a) and theamide (b) at a surface pressure of 25 mNm−1.

50

40

30

20

10

00.4 0.6 0.8 1.2 1.6

Area per molecule (nm2)

Surf

ace

pres

sure

(m

N m−1

)

Figure 25: Isotherm of the salt.

speed of 5 mm min−1 upon withdrawal from the water. Thedeposition trace suggested that there is an even depositionand the transfer ratio is about 0.75. Therefore, in the LBfilm monolayer the area occupied by one salt molecule wascalculated to be about 0.8 nm2. The difference in occupiedarea per molecule indicates that the chain leans slightly to thesurface normal. The tilt of the chains in the LB monolayer isinduced by the deposition process.

8.2. FTIR Study for LB Films. The friction properties may beenhanced by a microscopically smooth coverage of the lubri-cant film over the media surface. In the FTIR-RAS spectra,the components of the vibrational moments of the chemicalbonds, which are parallel to the substrate normal, selectivelyappear [79]. Therefore, the FTIR-RAS spectra are useful forinvestigating the molecular orientation of a lubricant film ona substrate.

The salt-type lubricant films, which are prepared by theLB method and the spontaneous adsorption from the lubri-cant solution, are compared in Figure 26. The bands in the3000–2800 cm−1 and 1370–1100 cm−1 regions are assignedto the CH and the CF stretching vibrations, respectively. Theband at around 1674 cm−1 is assigned to the COO− antisym-metric vibration. The absolute intensities of these bands arevery similar in both spectra, which suggest that the adsorbedlayer of the salt is in fact a monolayer and that the degree

3000 2500 2000 1500 1000

Tran

smit

tan

ce

0.2%

(a)

(b)

Wavelength (cm−1)

Figure 26: FTIR reflection adsorption spectra of the novel lubricantfilms prepared by (a) spontaneous adsorption method and (b) LBmethod on the surface of the magnetic layer.

(b) After the friction test2928

2928

2858

28581674

16741156

1156

1218

1218

1246

1404

1404

0.2%

(a) Before the friction test

Tran

smit

tan

ce

2918

2850

1674

115012061232

1404

1246

3000 2500 2000 1500 1000

(c) Bulk material

Wavenumber (cm−1)

Figure 27: FTIR spectra of ammonium salt, C7F15-COO−H3N+-C18F37 (Lubricant 5) (a) RAS spectra of adsorbed film (b) RAS spec-tra of adsorbed film after friction test (c) transmission spectrum ofbulk material.

of orientation and packing of the alkyl and perfluoroalkylchains closely resembles that in the LB monolayer.

9. Spectroscopy of Adsorbed Lubricant Film

9.1. FTIR Study for Lubricant Film of Different HydrophilicGroups. Figures 27 and 28 compare the RAS of the lubricantfilm and transmission spectra of the bulk material for the saltand the amide, respectively. The mode assignment and peakpositions for the RAS and for the transmission spectra aresummarized in Table 5.

For the salt, the differences in the spectra betweenthe RAS of the adsorbed lubricant film on the substrate


Table 5: Mode assignment and peak positions for the ammonium salt and amide in KBr and as adsorbed on a magnetic layer.

Peak position/cm−1

Amide Ammonium salt Vibration mode

Bulk Film Balk Film

2918 2928 2918 2918 CH2 asymmetric stretching

2850 2856 2850 2858 CH2 symmetric stretching

1674 1674 COO− asymmetric stretching

1404 1404 COO− symmetric stretching

1692 CO stretching

1232 1232 1246 CF2 asymmetric stretching (E1 symmetry)

1206 1206 1218 CF2 asymmetric stretching (A2 symmetry) + CF3 stretching

1148 1150 1156 CF2 symmetric stretching (E1 symmetry)

3000 2500 2000 1500 1000

(a) Before the friction test


2856

2918

2850

1692 15400.2%

29182850 1692

1540

1206

1206

1148

1148(c) Bulk material


2856

2918

2850

1692 15400.2%

29182850 1692

1540

120200022 6

12061148

(c) Bulk material

Wavenumber (cm−1)

Tran

smit

tan

ce

Figure 28: FTIR spectra of amide, C7F15-CONH-C18F37 (Lubricant3) (a) RAS spectra of adsorbed film (b) RAS spectra of adsorbedfilm after friction test (c) transmission spectrum of bulk material.

(a) and the transmission spectra of the bulk materials (c) aredescribed as follows.

(1) In the RAS spectrum of the adsorbed film, thebond assigned to the COO− asymmetric stretchingvibration at 1674 cm−1 is much weaker than in thespectrum of the bulk material.

(2) The relative intensity in wavenumber of the CF2

stretching vibrations in the 1250–1140 cm−1 region ischanged and shifted to a higher frequency in the RASspectrum.

(3) The band assigned to the CH2 stretching vibrationin the RAS spectrum in the vicinity of 2900 cm−1 isalso shifted to a higher frequency than that in thespectrum of the bulk.

The weakness of the band at 1674 cm−1 suggests thatasymmetric stretching of the COO− (hydrophilic) group

is almost parallel to the substrate surface in the adsorbedfilm. The changes in the relative intensity and the higherfrequency shift in the 1250 to 1140 cm−1 region are similarto those reported for a monolayer of perfluorocarboxylicacid [80]. These changes have been attributed to adsorptionof the molecules with a preferential orientation and theperfluoroalkyl chains tilted in the adsorbed layer.

The higher frequency shift of the CH2 stretching vibra-tions is often observed when the alkyl chains are in aliquid phase or in a solution, where the cohesive interactionbetween them is weak [66]. A probable cause of the higherfrequency shift in the RAS spectrum is that the perfluoroalkylchains hinder a cohesive interaction between the alkyl chainsin the monolayer.

For the amide, the spectral pattern of the film in theregion of the CF2 and CF3 stretching vibrations, 1250 to1140 cm−1, is different from that of the salt. However, thedecrease in intensity of the stretching vibrations for C=O at1692 cm−1 and the high frequency shift of the CH2 stretchingmode behaves similar to the carbonyl stretch and highfrequency CH2 shift for the salt.

The FTIR-RAS spectra of the lubricant film after the fric-tion test are also shown in Figures 27(b) and 28(b). The abso-lute intensity of the CH2 stretching region is similar in thespectra both before and after the friction test, indicating thatsliding causes no substantial change in the surface concentra-tion of the alkyl groups. For the salt, the spectra both beforeand after the test are similar, therefore, the rubbing motioncaused no change in the microscopic structure of the lubri-cant film. On the other hand, there are several differences inthe RAS spectrum of the adsorbed film of the amide beforeand after the test. These results are summarized as follows.

(1) The bands at approximately 1692 cm−1 and1540 cm−1, which are assigned to the C=O stretchingvibration and the NH bending vibration, respectively,appear after the test.

(2) The pattern of the CF2 and CF3 stretching vibrationsat 1250 cm−1 and 1140 cm−1 changes.

(3) The band of the CH2 antisymmetrical and symmet-rical stretching vibrations in the 3000 to 2800 cm−1

region shifts to a lower frequency.


Magnetic layer

Magnetic layer

Island model

Uniform modeld

d

Θ = σ1/σ1 + σ2

σ1 σ2

Figure 29: Lubricant coverage models. Θ is the lubricant coverageratio in island model and d is the average thickness on magneticlayer.

Thus, the spectrum of the adsorbed amide filmapproaches the spectrum of the bulk material with sliding.This suggests rearrangement of the amide monolayer by thesliding process, perhaps into three-dimensional crystals oramorphous piles.

These results show that the layers of the amide werecomposed of orientated molecules, although they produceda higher friction coefficient than those of the salt. A possiblecause for the higher friction coefficient is that a layer struc-ture with a high degree of molecular orientation is less stablefor the amide compared to the salt. This is demonstrated bythe decay curve on the water surface in Figure 24.

The FTIR-RAS spectra showed that the polar group ofboth lubricants interacted with the magnetic surface beforethe friction test. For the ammonium salt, the friction coeffi-cient is low and constant throughout the 100 reciprocatingcycles (Figure 9). From a spectroscopic point of view, beforeand after the spectra of the friction test are similar, suggestingno change in the structure of the lubricant film. In contrast,for the amide, the friction increases with the number ofcycles, thus the adsorbed lubricant was changed into bulk-like aggregates, which brings about the bare contact and leadsto a higher friction.

9.2. Angle Resolved X-Ray Photoelectron Spectroscopy(ARXPS) Study for Microscopic Coverage. The uniformityof the monolayer level lubricant on the magnetic thin filmmedia has been investigated using ARXPS with the findingthat the PFPE is a discontinuous film [50, 81]. Kimachiet al. proposed an island model to describe the coverageof conventional PFPEs on the magnetic recording media[82]. On the other hand, the modified PFPE has a goodlubricity compared to the conventional ones, which impliesa different film formation. In order to gain insight into thiseffect, microscopic coverage of the lubricant on the surface isinvestigated by ARXPS. The Z-lubricants with and withoutmodification are deposited under the same conditions.

A lubricant layer completely covers the surface with aconstant thickness of d in the uniform model. In the islandmodel, the surface is discretely covered with lubricant islandshaving average the thickness of d (Figure 29). It is assumed inthese models that the surface is flat and that elemental atomsare homogeneously distributed in the lubricant layer [83].Based on these assumptions, the photo-electron intensityratio, Ilub/Isub is expressed as a function of the take-off angle,

0.2 0.4 0.6 0.8 1

2

4

6

8

10

0

70%

60%

50%

30%

Modified Z-lube (lubricant 20)Z-lube

0

sinϕ

I lu

b/I

sub

100% coverage Θ

Figure 30: Coverage of the media surface with lubricant by ARXPSand the calculated curves (solid line) using island model shown forcomparison.

ϕ, and the lubricant coverage ratio, Θ, on the surface ofthe magnetic layer. Ilub is the intensity of the photoelectrondetected from the lubricant layer, and Isub is the intensityfrom the under layer.

In Figure 30, Ilub/Isub is plotted as a function of sin ϕ, andthe calculated model curves of Θ using the island model arealso shown. Because the IF of the modified PFPE is somewhatlow, Ilub/Isub is normalized in such a way that the Ilub/Isub at90 degrees is 1.

The coverage of the modified PFPE is greater than theconventional lubricant, despite the fact that the amount oflubricant on the surface is somewhat lower. The frictioncoefficient of the magnetic media coated with bondedlubricant film decreases linearly with increasing surfacecoverage [84]. The higher coverage reduces the dry contactsat the rubbing surface and minimizes friction.

It can be considered that two factors determine low fric-tion, namely, better coverage and a strong interactionbetween the lubricant and the media surface. Better coverageis ascribed to the balance of the hydrophobic and hydrophilicproperties of the lubricant. It is well balanced when thehydrocarbon chain is introduced. The polar group of theammonium salt without steric hindrance has strong inter-actions at the surface.

10. Physicochemical Aspect

10.1. Surface Energy. The surface energy is determined fromthe contact angle, θ, measurements using water and diiod-omethane. The methodology for these measurements was


Table 6: Calculated dispersion and polar contribution to surface energy based upon contact angle measurements.

LubricantContact angle/deg

γd/mJm−2 γp/mJm−2 γtotal/mJm−2

H2O CH2I2

Stearyl (9) 83.8 64.7 21.4 6.7 28.1

Oleyl (11) 83.9 67.4 19.7 7.3 27.0

Linolenyl (12) 82.9 73.2 16.1 9.4 25.6

Table 7: Heat of adsorption and desorption of the lubricants on the surface.

Structure and lubricant number Dielectric constant∗∗ Heat of adsorption/erg cm−2 Heat of desorption/erg cm−2

C7F15CO O C18H31 (2) 2.01 2.1 1.0

C7F15CO NH C18H31∗ (4) 2.36 2.7 1.9

C7F15 O−H3N+C18H31 (6) 2.66 18.9 Not detected∗Stearyl amide is slightly soluble in the solvent, therefore, the double bonded lubricants are used. ∗∗Dielectric constant is measured at 3 MHz.

described by Kaeble [85]; therefore, an extensive reiterationis not necessary here.

The contact angle is related to the surface energy viaYoung’s equation:

γS = γSL + γL cos θ, (1)

where γS is the surface energy of the solid, γL is the surfaceenergy of the liquid, and γSL is the interfacial energy ofthe solid and liquid. When the reference liquid is capableof interacting with the surface through both dispersive andpolar forces, the interfacial surface energy can be written asfollows:

γSL = γS + γL − 2(γSγL

)1/2

= γS + γL − 2[(

γdSγdL

)1/2+(γPS γ

PL

)1/2]

,(2)

where γdS and γPS are the dispersive and polar components ofthe solid surface energy, respectively (Schrader [86]). Also,γdL and γPL are the liquid surface energies. The dispersivecomponent is the London dispersion force contribution tothe surface energy, and the polar component is a measure ofthe polar interactions such as hydrogen bonding or acid-baseneutralization (Fowkes [87]). γdS and γPS are obtained by thesubstitution of (2) into (1).

The disorder of the hydrocarbon chains is also distin-guishable by the wettability, which is determined by thenature of the outermost atomic group of the lubricant layer.Table 6 summarizes the surface properties calculated fromthe contact angle measurements.

The surface energies are evaluated by Kaelble’s method[80]. As the adsorption model is shown in Figure 12,the unsaturated chains are bent and inefficiently packed,so that the perfluoroalkyl chains appear near the outsidesurface, and lower the surface energy value, which is mainlydetermined by the dispersive component.

10.2. Heat of the Preferential Adsorption. These three types oflubricants, the salt (Lubricant 5), the amide (Lubricant 3),and the ester (Lubricant 1), have the same hydrophobicgroups, but have different friction coefficients, which is

related to the polar group of the lubricant molecule. Aplausible explanation is that the salt-type lubricant leads togreater adhesion than the ester and amide. As the surface ofthe sputtered carbon protective layer is classified as ratherpolar because it contains 5 and 7 atom % hydroxyl andcarbonyl groups, respectively, a source of the attractive forceat the surface may be the interactions between these polargroups [63]. In order to gain insight into this effect onthe friction properties, the dielectric loss measurements areconducted.

The relative dielectric constant (ε) of the lubricantsis measured at 3 MHz and summarized in Table 7. Therelative dielectric constant, which is a parameter of thedipole moment, is 2.01 and 2.36 for the ester and theamide, respectively, but 2.66 for the salt-type. The higher thedielectric constant, the more strongly the lubricant seems toadsorb on the medium [88].

The formation of the lubricant film is a spontaneousprocess caused by a decrease in the free energy of the solidsurfaces and lubricant molecule adsorption. The heat ofadsorption of lubricants on a rubbing surface can be takenas a measure of the strength of attachment of the lubricantmolecules to the surfaces and are also shown in Table 7.

For the ester and the amide-type, the heat of adsorptionis small, and the heat of desorption is partially observed.However, for the salt-type, the heat of adsorption was veryhigh compared to the ester and the amide, and the heatof desorption could not be detected. These results confirmthat the ester and the amide were held by weak interactionson the carbon surface, whereas irreversible adsorption hadtaken place for the salt type. The high heat of adsorption ofthe salt type on the carbon surface results from these stronginteractions and accounts for its low and steady friction.The low heat of adsorption for the ester and the amide, bycontrast, apparently produces a film which is easily disruptedby sliding to give a rising friction coefficient with sliding.

11. Conclusions

The frictional properties of newly synthesized ionic liq-uid lubricants for magnetic media have been investigated.


A novel ionic liquid, which has an ammonium salt with acarboxylate as a hydrophilic group, has a lower frictionalcoefficient than the other conventional PFPE. It can beconsidered that two factors determined the low friction,namely, better coverage and strong interaction between thelubricant and the media surface.

These ionic lubricants invented around 1987 [26, 44, 89–91] have been used for magnetic tapes of 8 mm video, digitalvideo cassette, the AIT system, and the broadcast applicationfor about a quarter century because of their good lubricityand also from an environmental point of view.

The effects of the molecular structure of the modifiedPFPE on the frictional properties are summarized as follows.

(1) Stronger adsorption due to the adhesive interactionof the polar group, thus the novel carboxylic acidammonium salt has a lower and more stable frictioncoefficient.

(2) Sufficient length and symmetry of the hydrocarbonchain cause extensive cohesive interactions, and thesedispersive interactions compensate for the frictionreduction.

(3) On the contrary, for the olefinic lubricant, the fric-tion coefficient increased with the increasing numberof reciprocating cycles because of the weaker cohesiveinteractions. The introduction of a double bond atthe terminal position affected an increase in frictioncompared to the center position.

(4) The ammonium salt polar group can be introducedto the commercial PFPE, and the frictional coefficientis independent of the molecular structure of the PFPEbackbone polymer and of the thinner dip-coating,which makes practical use convenient without anyenvironmental problems.

From a microscopic point of view, the lubricant filmcoverage was also investigated as follows.

(1) A layer of the carboxylic acid ammonium salt lubri-cant film prepared by the spontaneous adsorptionis highly ordered and closely packed as in the LBmonolayer.

(2) The polar COO− groups of the salt are adsorbedalmost parallel to the surface, and the slidingscarcely changes the salt-type lubricant film, whilethe arrangement of the amide film into a bulk phaseoccurs due to the sliding process.

(3) The modified PFPE uniformly covers the magneticsurfaces; this is why it minimizes the friction.

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